When I got a supercharger kit for my morris minor, it came with a sacrificial belt to cut and then measure the gap fitted, think was an 6pk1100 belt so for example if it was 150mm gap, you'd get a 6pk1250 (or slightly longer for adjustment) or whichever is nearest size as its not necessarily specific increments. The 6pk is 6 peaks, 4pk is 4 peaks etc, then length after. But string is more flexible so probably easier on a serpentine route.
also worth measuring the belt you have to see if it actually matches the measurements (and if it does, where the measurement is made from)
Apologies for the tardy update... 735mm belt arrived a few days ago and put it on this AM. Engine's been put back in since the original post, so was expecting it to be an even bigger pain to fit than previously. Pleasantly surprised, even went on without having to remove any pulleys like before. Found that as long as all the various fixing points for the PAS pump were loosened off enough that its pulley would tilt out a few mm at the bottom and the brackets adjustment screw was at it farthest, the belt slipped over with minimal pushing. Before the PAS and water pump pulleys were pressing into one another at this point, now there's a gap of a few mm and that's before it'll be opened further as things are tightened up. That 5mm seems to make all the difference!
That's good. I've always had to remove the PAS pulley, fit the belt to it and then screw the pulley back on. Perhaps I'll try a slightly longer belt next time.